In the vacuum semiconductor wafer processing field, layout of the various system components such as load locks, process chambers, intermediate processes (e.g., pre-clean or cooldown) and transfer mechanisms (e.g., robots or conveyors) is critical to both system cost and reliability, as well as to footprint and productivity. Optimal component layout reduces wafer processing costs by eliminating the need for costly multi-axis wafer handlers, by reducing footprint and cleanroom costs associated therewith, by reducing the cost associated with non-value added wafer transport time, and by increasing reliability. Accordingly, much attention is directed to optimizing fabrication tool configuration so as to reduce the fabrication tool's footprint, and to simplify the wafer transfer process.
A conventional fabrication tool configuration is disclosed in U.S. Pat. No. 4,722,298 entitled "Modular Processing Apparatus for Processing Semiconductor Wafers," (the '298 patent). The '298 patent teaches a modular semiconductor wafer processing apparatus comprised of a plurality of modules. Each module has a chassis, a process chamber, a connection means for releasably connecting a service supply, and a rotational pick and place robot arm for extracting wafers from the process chamber. To form a modular semiconductor wafer processing system a plurality of the modular units are aligned such that the rotational pick and place robot arm of a first module picks up a first wafer from the first module and transports it to the second module where it is deposited for further processing. After processing is complete within the second module the rotational pick and place robot of the second module picks up the wafer and rotates, carrying it to the process chamber of a third module for further processing.
The '298 patent does not provide a detailed description of the process chamber configuration and of the robot arm operation. Presumably the process chamber would have two ports located on opposite sides of the chamber, an extraction port and an insertion port. In operation the rotational robot arm rotates to position itself in front of the extraction port of a first process chamber, the port opens and the robot arm extends, reaches into the process chamber, picks up the wafer, retracts and the port closes. The robot arm then rotates to position itself in front of the insertion port of a second process chamber. The port opens, the robot arm extends, depositing the wafer within the second process chamber, and then retracts. In this manner a wafer passes in one port of a process chamber, through the process chamber and out the port on the opposite side of the process chamber. Each process chamber is thereby exposed to ambient atmosphere (i.e., the atmosphere of the room in which the '298 semiconductor processing apparatus is located) during wafer transfer.
In order to move a wafer from one processing chamber to the next, many time consuming steps are necessary: (1) a first process chamber is most probably vented to ambient atmosphere; (2) the first process chamber's extraction port is opened; (3) the wafer is removed from the first process chamber; (4) a second process chamber's insertion port is opened; (5) the wafer is loaded into the second process chamber; (6) the second process chamber's insertion port is closed; and (7) the second process chamber is pumped down to the vacuum level required for processing. A wafer is therefore exposed to contaminants from the ambient atmosphere each time a wafer is transferred. Further, each time chamber pressure is altered, stationary particles can mobilize and therefore increase wafer contamination.
The modular configuration of the '298 patent advantageously allows a module to be quickly and easily replaced, thereby reducing downtime costs; and allows a fabrication tool to be easily reconfigured as processing requirements change. However, the '298 patent's configuration is strictly a serial wafer processing system--a wafer can only move from one processing module to the next adjacent module. Accordingly the '298 patent is limited to performance of a single processing sequence, any sequence change requires the entire apparatus to be shut down while processing modules are rearranged.
Further, the configuration taught by the '298 patent requires the use of a rotational pick and place robot within each module. Such robots are more expensive, less precise and require larger operating footprints than do conventional linear robots. The risk of wafer breakage, misplacement, etc., is also greater when using rotational robots than when using conventional linear robots. As well, the configuration of the '298 patent requires non-conventional, two-port process chambers and requires wafers to be transferred from a first wafer carrier to a second wafer carrier.
Because each wafer within a semiconductor fabrication facility is tracked by wafer carrier and wafer carrier slot number, a wafer which is not returned to its original carrier and its original slot becomes "lost." Therefore in practice, the system disclosed in the '298 patent would require an external mechanism to move wafers back to their original wafer carrier and slot (increasing processing time, equipment costs and particle generation).
Accordingly, a need exists for an improved modular semiconductor device fabrication system that allows quick configuration and repair, that is less expensive, more precise, and smaller in footprint than conventional modular systems, and that is easily constructed using conventional process chambers and transfer mechanisms. Such a fabrication system should provide random access to processing chambers, thus enabling performance of a number of processing sequences, and should be capable of maintaining wafer carrier and wafer carrier slot integrity.